1. Field of the Invention
The present invention relates to a device for detecting information in which information recorded on a recording medium at a high density can be detected with no error.
2. Description of the Related Art
Research and development on disk units with high densities and large capacities such as optical and magnetic disks represented by DVDs is widely and energetically being pursued. Signal processing technology supporting high reliability in regenerated information is indispensable for such disk units with the higher density features. To this end, filing devices using PRML (Partial Response Maximum Likelihood) technology have successfully entered the market. This system, in which equalization of a partial response wave form and maximum likelihood detection are combined, is well known by conducting the maximum likelihood retrieval after correcting readout signals by wave form equalization in order to utilize characteristics of a maximum likelihood detector giving full consideration to a regenerative channel. For example, there is a description on PRML in the preliminary paper volume for the 1994 annual convention of the Institute of Television Engineers (ITE '94, pp. 287 to 288).
Intersymbol interference grows larger and regenerative amplitude is reduced in the case where information recorded with a high density is regenerated in either an optical disk or a magnetic disk. Therefore, the SNR of a magnetic disk is smaller and the CNR of the high frequency component of a readout signal in the case of an optical disk is also smaller, so that the error rate of detected information is increased. The maximum likelihood detection system detects information by using characteristics of a regenerative channel having a given state transition and if one time series pattern is screened so as to have the minimum square mean of errors among all the time series patterns to be conceivable from the characteristics of the regenerative channel with respect to an amplitude information string having the number of quantized bits, for example, on the order of four bits, which is input to the detector, information can be detected with a low error rate, even though the SNR or CNR is small. It is difficult, however, to perform the above mentioned processing in an actual circuit in terms of the circuit scale and its processing speed, and thus an algorithm called a Viterbi algorithm is used to select a path in a recursive manner in order to realize the process, wherein the Viterbi algorithm is described in IEEE Transactions on Communications, vol. COM-19, October 1971.
An operation in the case where information recorded in an optical disk medium is detected by being subjected to PRML detection using the most simple PR (1, 1) channel is described with reference to FIGS. 21, 22, and 23. A signal read by a head is corrected in advance so as to be the PR (1, 1) channel by use of an equalizer, for example a transversal filter. As shown in FIG. 21, this channel is distributed around three reference levels E1 (=-1, 0, 1). In this case, amplitude information X.sub.i which is digitized in each channel clock is subjected to a two state transition as shown in FIG. 20. A maximum likelihood detection finds a string Ei with which a sum of square errors z.sub.n relative to a reference level shown in the mathematical formula 1 is the minimum with respect to x.sub.i. ##EQU1##
However, it is hard in an actual time span to obtain Ei with which z.sub.n is the minimum by calculating the equation (1) in every case conceivable. Thus, Ei is generally determined using a procedure called a Viterbi algorithm. A graph shown in FIG. 23, which is obtained by redrawing the state transition diagram of FIG. 22 on an abscissa of time, is called a trellis diagram. In a Viterbi algorithm, z.sub.n values of two paths are calculated from respective sums till a time n-1 and x.sub.n input at a time point n and then the path which has the smaller value of z.sub.n is selected, wherein the term "path" means a directive graph in which a time point and the next time point are connected by a line. As time is retroacted from the present to the past, while this path selection is repeatedly conducted at each time point, paths are converged at a time point expressed by an expression indicating that the paths have been merged. This means that it has been determined that a single path is left and the output corresponding to the path is the detection result. Generally, the z.sub.n is called a path metric, and the path metric at a time point is called a branch metric. EQU M.sub.n (S.sub.0)=min [M.sub.n-1 (S.sub.0)+(X.sub.n +1).sup.2, M.sub.n-1 (S.sub.1)+X.sub.n.sup.2 ] EQU M.sub.n (S.sub.1)=min [M.sub.n-1 (S.sub.1)+(X.sub.n -1).sup.2, M.sub.n-1 (S.sub.0)+X.sub.n.sup.2 ] (2)
The mathematical formula (2) is a recurrence formula in the case where the above mentioned algorithm is adapted to the state transition of FIG. 22. Mn (S.sub.j) indicates a sum z.sub.n of square errors until a time point n in a state S.sub.j at the time point n and min [a, b] is a function indicating the minimum between a and b. Path metric values for two respective paths at an immediately preceding time point when inputs are respectively made to states S.sub.0 and S.sub.1 are calculated, whereupon the smaller one is selected and thereafter the path metric value is renewed. FIG. 23 illustrates an example in which selection of a path is conducted at each time point using the mathematical formula (2). A thick line indicates a path conceivable at a time point. Two paths, respectively thick and thin, are present between time points 0 to 6 and there is no determined path, but a merged path is present at time points of 7 and 9. After merging, an output value q.sub.l corresponding to the path is successfully output and thereby the most likelihood detection can be performed.
In order to actually constitute a circuit, (y.sub.n +1).sup.2, y.sub.n.sup.2 and (y.sub.n -1).sup.2 are generated in a branch metric calculating circuit 21, such as shown in FIG. 4. Subsequently, path metric values Mn (S.sub.0) and Mn (S.sub.1), and the branch metric value are added, a comparative calculation is conducted and thereby one of the paths input to a state is selected and further the path metric value of the path is used as the new path metric value. This operation is repeated and thereby paths are merged into a single path to detect the most likely path. A comparator output indicating selection information of a path is stored in a path memory 23 and detection of information can be conducted by outputting bit information corresponding to a determined path before a merging point.
In a rewritable optical disk device in its first generation, a mark position recording system is adopted, wherein data is recorded on a disk medium by being converted to a length between pits and information is detected by a peak detection system. In peak detection, a readout signal is differentiated and subjected to zero-cross and thereby the middle position of a recording pit is detected, so that a level fluctuation has never been problematic. When a high density in recording is used, a regeneration amplitude is reduced in the mark position recording system and thereby information cannot be detected with high reliability. For this reason, the mark edge recording system as used in CDS, in which a length of a pit itself bears information, has begun to be used in optical disks. However, when information recorded in this recording system is regenerated, since a peak detecting system cannot be used, a level detection is generally used in which 0 and 1 is decided with a threshold value as a reference. In this case, level fluctuation directly gives a wrong influence and it can be said that this level fluctuation is one of the significant causes for disturbing the process of achieving a higher density.
Especially in the case of an optical disk, a direct current level included in regenerated information has a chance to fluctuate.
(1) There is a possibility, when a polycarbonate substrate is used, that noise up to the order of 10 kHz is mixed into the regenerated information by the influence of birefringence.
(2) In a code file which is subjected to sector segmentation, random access is made possible by providing a preformat region in a leading part of a sector, but this preformat information has a large direct current component and thereby data following the preformat is affected.
(3) A RLL (Run length limited) code adopted as a modulation code is subjected to level fluctuation depending on the record pattern, since the code is not DC free.
In a similar manner to the level detection system, when Viterbi detection is conducted, a direct current level fluctuation causes great degradation in performance as well. In general, while a Viterbi detector has a circuit constitution corresponding to a fixed regenerative channel because of a scale down of the circuit, channel characteristics expected in the Viterbi detector are not realized, since the information string input to the Viterbi detector is added with an offset by a direct current level fluctuation. The reason why is that the addition of the offset is equivalent to superposition of noise on a regenerated information and thereby the reliability in detection is degraded by a great margin.
Conventionally, there have been proposed many techniques to avoid degradation in a performance of Viterbi detection caused by direct current fluctuation. For example, in a system disclosed in Publication of Unexamined Japanese Patent Application No. Hei 6-325504, degradation in performance of Viterbi decoder by a direct current fluctuation is sought to be prevented. FIG. 1 shows a conventional technique described in the publication. There is described that, in an optical disk device in which it is made a precondition that Viterbi detection is conducted under application of a class 1 partial response, a transition pattern, which is changeable when a signal level of a regenerative signal traverses a center level, is detected and the center level is corrected using a sample data in the pattern, so that even when a direct current level is fluctuated, the regenerative signal can correctly be decoded.
Another system shown in Publication of Unexamined Japanese Patent Application No. Hei-7-45009, aims to solve its poor convergence property in a common adaptive control of a level fluctuation. FIG. 2 shows a conventional technique described in the publication. The system comprises a pattern generating means 90 in which transmitted data is divided into block units and a test pattern is added at the leading part of each block, and a switching means 80 in which transmission is conducted by switching pattern data to transmitted data, wherein a known test pattern is added at the leading part of each block and transmitted or recorded. In an operation of regeneration, a direct current component of a transmitted signal is calculated by a predictive control from a transmitted signal of a test data portion taken out by switching means 70 for switching between test data and actual data. There is described a technique that this direct current component is added to a reference predictive amplitude value of a Viterbi decoder 50 to obtain an initial predictive amplitude value and further a small level fluctuation is compensated by predictive control means 60 which controls in a predictive manner.
In a system shown in Publication of Unexamined Japanese Patent Application No. Hei 7-262694, as well, it is an object to prevent degradation in performance of a Viterbi decoder caused by a direct current level fluctuation. FIG. 3 is a conventional technique disclosed in the publication. There is described a technique in which a predetermined sample value in a sample value string is extracted by data discriminating means 16 and sample extracting means 13 using a data amplitude value q after A/D conversion of a readout signal and averaged to detect an offset level. Further, the offset level is uniformly added to each of the predictive sample value in the Viterbi decoder 20, so that a direct current level fluctuation is compensated in an adaptive manner.
A first problem in the conventional technique disclosed in Publication of Unexamined Japanese Patent Application No. Hei 6-325504 is that the technique cannot be applied to other channels, since it only refers to a class 1 partial response. For example, when it is applied to the case where high density recording is conducted, there is presented a simulation result in which a PR (1, 2, 2, 1) channel can detect information with higher accuracy than a PR (1,1) channel can, because of reduction in resolution, as reported in the preliminary paper volume of the Institute of Electronics, Information and Communication Engineers, C-469. A second problem is that the technique detects amplitude information only from transition patterns traversing a center level and for this reason, a SNR of a detected direct current level is low. Since a direct current value must be added to all input information, accuracy is reduced, compared with the case where all information is utilized to detect the direct current level value. A problem with the conventional technique disclosed in Publication of Unexamined Japanese Patent Application No. Hei 7-45009 is that format efficiency is reduced, since there is a necessity that a special pattern is recorded at the leading part of each block. A problem with the conventional technique disclosed in the publication is that the SNR level of a detected direct current level is low, as in the case of the publication of Unexamined Japanese Patent Application No. Hei 6-325504, since only special sample values are extracted and averaged.